1 in 2000 Britons may carry 'mad cow' prion protein

"We were all supposed to die of mad cow disease!" People who accuse public health agencies of crying wolf are fond of citing the discovery, in 1996, that a cattle disease widespread in the UK causes the deadly disease vCJD in people. Despite widespread dismay at the time, there have been only 177 cases of vCJD in the country – and 51 elsewhere – to date.

The biggest survey yet, however, shows the UK did not really dodge that bullet – it just hasn't killed many of the people it hit. What we don't know is how many might still die.

A study of 32,000 appendixes removed between 2000 and 2012 from British people born between 1941 and 1985 suggests that 1 in every 2000 people in the UK is carrying the abnormal protein, or prion, that causes the disease. This means as many as 31,000 people may carry the prion – twice the previous best estimate.

The researchers, mostly from the UK's national human and animal health labs and led by Sebastian Brandner of University College London, warn that we do not know what further damage those infections may cause. In particular, there seems to have been less transmission of the prion via blood transfusions than would have been expected. The researchers are calling for development of a reliable blood test for the prion so we can make sure it is not spreading undetected.

Silver lining

Half the people infected are at particular risk: they carry the genetic form of the protein that has been found in all cases of vCJD to date. However, the researchers warn that they do not know whether such people will simply be lifelong carriers, or may one day develop vCJD. Meanwhile, other genetic forms of the prion could be affecting people in unrecognised ways.

The silver lining, says Richard Salmon, a retired neurologist who wrote an editorial accompanying the research, is that recent research shows that the vCJD prion behaves much like the pathological proteins behind a number of other diseases involving brain degeneration, including Alzheimer's and Parkinson's diseases. These are huge threats to ageing populations.

"We have developed, of necessity, a huge body of expertise in studying prion diseases in Britain," says Salmon. This knowledge could be used to study treatments for such things as Alzheimer's. However, he fears as worries about vCJD wane, funding to maintain that expertise is waning too.

How it happened

Bovine spongiform encephalopathy (BSE) is caused by a misfolded protein – a prion – which accumulates in brain tissue, causing death. It is spread when susceptible animals eat tissues from infected animals that contain the prion. BSE was discovered in British cattle in 1987 and has been blamed on the widespread use of cattle remains as cattle fodder in the UK. The UK government initially claimed the prion could not spread to humans – but it was found to do so in 1996.

By that time inadequate controls meant infected beef had been in the human food chain for years, and there were fears of a mass plague of agonising, invariably fatal vCJD. Fortunately, they did not materialise, but it was unclear whether that was because the prion had not infected people or because for some reason it didn't make them sick.

A decade ago it was discovered that the prion lodges in the appendix, offering a way to search for it in living people who have their appendix removed. Early studies suggested widespread infection was possible, but the samples were too small to be sure.

Where we are now

Now it is clear that people of all ages and across the country were widely infected, regardless of whether they had the genetic form of the prion protein associated with the disease. In fact, more people with the alternate, resistant forms were found to be carrying the prion than would be expected if its distribution was random, for reasons unknown.

"This shows we need to understand more about the natural history of the prion," says Salmon. "Infections don't lead to disease as readily as we originally feared, but we don't know why, or whether these infections have a sting in their tail."

Models suggest infected people could still develop vCJD in coming decades. Salmon worries that the prion might cause diseases in people with the resistant genotypes that do not look like classic vCJD and so could be missed. This is especially likely in older people, in whom dementia is more common and not often investigated after death.

Meanwhile we could soon discover even more precisely who was infected during the days of BSE in the UK. Tests for prions in blood are almost ready. Markus Moser, CEO of Prionics, a BSE test manufacturer in Zurich, Switzerland, says his company and the National Institutes of Health in the US have developed the eQuIC assay, which detects prions at low enough levels that "in hamster and sheep, it works as a blood test".

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